1. Field of the Invention
The present invention relates to cooling actuator coils, and more particularly, to coils used in Lorenz force actuators and motors used in lithographic tools to levitate and drive stages.
2. Related Art
A Lorenz force actuator consists of two physically separate magnetic components. The first component is typically a drive coil through which a current is passed. The second component consists of an assembly of permanent magnets, often in combination with additional high permeability material, that together generates a strong magnetic field that passes through the coil. The interaction between the current flowing through the coil and the magnetic field generated by the permanent magnet assembly produces both a force on the coil, and an equal and opposite reaction force on the permanent magnet assembly. The portion of the coil in the strong magnetic field can be referred to as the active portion of the coil. Heat dissipated in the coil can cause an excessive rise in temperature, unless the coil is adequately cooled by conducting heat to a circulating cooling fluid.
In some cases, thin cooling jackets attached to the faces of the active portion of the coil are required to augment the flow of heat to the cooling fluid. Such cooling jackets are typically made of ceramics, such as silicon nitride, that offer a good combination of both high electrical resistivity and heat conductivity, and therefore efficiently transfer heat without causing undesirable eddy current damping effects due to the strong magnetic field.
Stainless steel is also used, but has neither high electrical resistivity nor high thermal conductivity. Fill polymers have high resistivity, but relatively poor thermal conductivity, and cannot, in general, be used to seal cooling water. Ceramics are brittle, expensive to manufacture, and might not (especially for thin sections) reliably carry the water without leakage.